A number of processes have been proposed for the preparation of methyl chloride from reaction of hydrogen chloride with methanol. Reaction is generally carried out in a vapor or liquid phase in the presence or absence of a catalyst. Most known processes use catalysts for promoting reaction and carry out reaction in an atmosphere containing a stoichiometric excess of hydrogen chloride relative to methanol.
However, methyl chloride production processes including the above-mentioned ones, which are currently implemented in the industry, have several drawbacks. The process of effecting vapor-phase reaction in the presence of a catalyst is disadvantageous in that reaction temperatures as high as 300 to 450.degree. C. cause more amounts of dimethyl ether and other decomposed products to be formed than other processes and requires the catalyst to be replaced or regenerated. This process requires gaseous hydrogen chloride which is anhydrous or dried to a nearly anhydrous extent and gaseous methanol as the reactants.
The process of effecting liquid-phase reaction in the presence of a catalyst is disadvantageous in that the reaction temperature is higher than in the liquid-phase non-catalyzed process, more dimethyl ether by-product is formed due to the use of the catalyst, and the catalyst must be periodically renewed because impurities accumulate in the catalyst.
While hydrogen chloride by-product obtained in a silicone manufacturing plant contains silicones and hydrogen chloride by-product resulting from other processes contains organic impurities, the use of such hydrogen chloride by-product as the reactant is not suitable for the catalyzed vapor- or liquid-phase processes because the catalyst can be deactivated by silicones or other organic impurities. A limit is imposed on the reactants which can be used in these processes.
The process of effecting liquid-phase reaction in the absence of a catalyst suffers from a low reaction rate. Since methanol is reacted with an excess over stoichiometry of hydrogen chloride, the concentration of hydrogen chloride in the reactor exceeds the azeotropic concentration so that a substantial fraction of hydrogen chloride may be distilled off the reactor along with the methyl chloride gas produced, leading to a noticeably reduced utilization of hydrogen chloride.
Illustratively, U.S. Pat. Nos. 3,981,938, 4,220,609 and 4,922,043 describe a process for forming methyl chloride by reacting methanol with a stoichiometric excess or equivalent amount of hydrogen chloride. This process has the above-mentioned disadvantage that the concentration of hydrogen chloride in the reactor approaches the azeotropic concentration so that a large fraction of hydrogen chloride may be evaporated from the reactor along with the methyl chloride gas produced, leading to a noticeably reduced utilization of hydrogen chloride. An aqueous solution of unreacted hydrogen chloride in water by-product can be separated from the crude product gas by cooling, but this solution is corrosive and very difficult to treat, requiring an additional expense for disposal.
U.S. Pat. No. 4,935,564 discloses a process for preparing an alkyl halide by passing a hydrogen halide and a stoichiometric excess (typically about 20 to 200 mol % excess) of an alcohol in the absence of a catalyst in a single pass through a plug-flow reactor. As the amount of alcohol is increased (in stoichiometric excess relative to hydrogen halide), the amount of dimethyl ether by-product increases. As the amount of alcohol is reduced, the conversion of hydrogen chloride declines. It is difficult to determine optimum reaction conditions. A single stage of plug-flow reactor is insufficient in order to suppress formation of by-products and to increase the conversion of hydrogen chloride.
U.S. Pat. No. 5,202,512 describes a process for preparing a C.sub.1 -C.sub.4 halogenoalkane by reacting a C.sub.1 -C.sub.4 alkanol with a hydrogen halide in the presence of a catalyst. On account of the catalyst used, this process has drawbacks including increased by-products, attachment of an extra equipment for adding the catalyst, difficulty to control the catalyst concentration, deactivation of the catalyst by impurities in the starting hydrogen chloride, and treatment of the deactivated catalyst.